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A 4 cylinder engine on a dyno at 1500 rpm.
Set the dyno load at 20hp, the amount of fuel burned to maintain that load is considered as 1 unit of consumption. Efficiency is low because you must restrict the airflow through the manifold.
This reduces the volume of air entering the cylinder which reduces compression and efficiency.
Now increase the load to 50 hp. Even though you are producing 150% (50hp versus 20), the fuel consumption only increases by 50%, or 1.5 parts of fuel for 50 hp versus 1part for 20.
50/1.5 = 33.33 hp per unit
20/1 = 20 hp per unit
Since each molecule of fuel is producing 33 units of work versus 20 units of work, lets understand why.
Increasing the load means reducing the manifold vacuum to produce the increased amount of work. This means more air, and more fuel for each combustion event.
The "all or nothing" tactic of pulse and glide (engine off or on) will give you the highest loads and efficiency, storing energy in the velocity of the vehicle while using 0 fuel in the glide (assuming engine off) or very small amounts (assuming engine on).
Doubling the thermal efficiency of the engine by not accepting inefficient operational parameters, can double your mileage.
Once that is understood then average speed becomes the critical factor. I could average 70 mpg instead of 55mpg if I choose to accept lower average speeds.
regards
mech
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